Recent evidence has been collected by a variety of investigators defining a distinct population of dendritic cells (Langerhans cells) of mesenchymal origin residing in the epithelial surfaces of many mammalian species. These cells play a dominant role in the processing of antigens presented through cutaneous surfaces, and carry a class II histocompatability antigen felt to be of central importance in the afferent arm of homograft rejection. They also possess many of the characteristics of macrophages active in the efferent arm of immunologic responses. Preliminary observations in our laboratory suggest an equivalent subset of dendritic cells (Langerhans cells) in the epithelium of the human, mouse, and guinea pig ocular surface. Our initial studies utilizing electron microscopy, ATPase staining and immunofluorescence indicate that these cells increwase in number in the corneal epithelium in response to injury, inflammation, and contact hypersensitivity. We propose a series of experiments, therefore, to define the anatomy, histochemistry, physiology, and immunology of these "ocular surface Langerhans cells." Their embryology, distribution and density in the ocular surface of the mouse, rat, guinea pig, and human will be assessed by electron microscopy and enzyme histochemistry. Membrane receptors and antigens will be identified by immunoflourescence with monoclonal antisera. Changes in distribution, density, and membrane antigens/receptors in response to corneal inflammation, chemical irritation, mechanical trauma, vitamin A deficiency, and ultraviolet light exposure will be identified. Ocular surface Langerhans cells will be propogated in tissue culture to assess their secretory activity (e.g. lysozyme, plasminogen activator, lkympokines). These studies will elucidate certain aspects of ocular allergy (especially contact hypersensitivity), marginal corneal ulceration, and corneal homograft rejection.